1. Field of the Invention
This invention relates to silicone compositions which have electrical conductivity which is unsually high and consistent from lot to lot.
2. Background Information
Silicone elastomeric compositions have been made electrically conductive by admixing conductive particles, such as silver particles and various forms of carbon black.
An electrically conductive, heat-producing composition was disclosed by Hunter in U.S. Pat. No. 3,099,578, issued Jul. 30, 1963, in which specific combinations of carbon, silver, and iron particles were mixed into a binder to give pigment concentrations of from 22 to 53 percent of the mixture.
Compositions comprising a mixture of polydiorganosiloxane and greater than 15 percent by weight of electrically conductive carbonaceous particles is taught in U.S. Pat. No. 4,250,075, issued Feb. 10, 1981. The compositions contain greater than 2 percent by weight of phenyl radicals.
Electrically conductive curable liquid organopolysiloxane compositions are disclosed in U.S. Pat. No. 4,273,697, issued Jun. 16, 1981 in which a liquid polyorganosiloxane, crosslinking agent, and a carbon black whose specific surface area is at least 900 m.sup.2 /g and which are hollow-shell-shaped particles. It may also contain a polydiorganosiloxane having hydroxyl groups at both ends of the molecule.
Graphite fibers were added to an electrically conductive silicone elastomeric mixture in U.S. Pat. No. 4,279,783, issued Jul. 21, 1981. The mixture is intended to be coated on a base member and used as an electrically conductive device, as an automotive ignition cable, for example.
An electrically conductive organopolysiloxane elastomer containing lamp black comprises from 15 to 60 percent by weight furnace black and from 5 to 25 percent by weight acetylene black is described in U.S. Pat. No. 4,387,046, issued Jun. 7, 1983. The resistance increases substantially as the composition is heated. The lowest electrical resistance shown is 110 ohm.cm.
The majority of the past work done with conductive silicone rubber has been intended for use as heating tapes or such and as controlled conductivity wires for ignition cable.
The present work was designed to produce as high an electrical conductivity, or conversely, as low an electrical resistance as possible. The discovery of the present method may make the following references of interest.
U.S. Pat. No. 3,824,208, issued Jul. 16, 1974 teaches a process for forming a free-flowing particulate polymer mixture from a viscous tacky polymer. Included in the mixture are silicone polymers and fillers such as fume silica, both treated and untreated and extending fillers such as carbon and graphite.
A liquid injection moldable silicone composition is described in U.S. Pat. No. 4,032,502, issued Jun. 28, 1977, containing silicone polymer, treated reinforcing silica filler, and a cure system of silicon hydride crosslinker and platinum catalyst.
U.S. Pat. No. 4,299,736, issued Nov. 10, 1981, discloses conductive, thermoplastics molding compositions. The electrical conductivity is improved by treating the conductive carbon particles used with a fatty acid chloride.
U.S. Pat. No. 4,469,522, issued Sept. 4, 1984, teaches a process for treating fillers with a fluorosilicone compound for use in fluorosilicone vulcanizable silicone rubber.
Fluorosilicone treated silica fillers are taught in U.S. Pat. No. 4,529,774, issued Jul. 16, 1985. The treated fillers are taught to be especially compatible with fluorosilicone gums.
The above references teach various methods of making polyorganosiloxanes conductive to electricity to varying degrees, but there is a need for an improvement in the conductivity, particularly in uses where the composition is to be used to transmit electrical current, rather than where it is used as a source of heat upon passage of an electrical current.